1. Field of the Invention
The present invention relates to a video tape recorder (VTR) equipped with reel driving apparatus.
2. Description of the Related Art
FIG. 1 is a view of an example of a conventional tape transport system with a tape loaded. In this diagram, the numeral 1 indicates the supply reel, numeral 2 indicates the take reel, numeral 3 indicates the reel motor, numeral 4 indicates the gear for transmitting the driving power of the reel 3 to the reels 1 and 2, numeral 5 indicates the drum, numeral 6 indicates the capstan, numeral 7 indicates the pinch roller and numeral 8 indicates the tape. The take reel 2 has a torque limiter which is between the drive source and the drive destination which makes use of, for example, friction, so as to prevent the tension F of the tape 8 from being excessive. When the tape 8 is moving forward (FWD), by rotating the reel motor 3 in the clockwise direction, the gear 4 will interlock with the take reel 2 so as to make it rotate in the clockwise direction. On the other hand, when the tape is moving in the reverse (RVS) direction, by rotating the reel motor 3 in the anti-clockwise (CCW) direction, the reel 4 will interlock with the supply reel 1 and the reel 2 will rotate in the anti-clockwise direction.
So, by maintaining a constant tape tension F at the part of the tape P when the tape is in transit, an output can be obtained from the desired head, tape damage such as that caused by the tape guides can be removed so that the reliability of the tape path can be increased.
Conventionally, in order to maintain a constant tape tension F, a mechanical tape tension regulator for controlling the tape tension was used with the forward (FWD) system and control of the tape tension in the reverse (RVS) system was carried out using a current servo system for controlling the current flowing to reel motor 3 which drives the reels in such a manner that it corresponds to the diameter of the tape roll.
As it is known, there is a directly proportional relationship between the current I flowing to the reel motor 3 and the torque generated by the motor, such that the equation 1 can be obtained using the tape roll diameter R, the tape tension F and the torque generated by the reels T.sub.r. EQU I.times..mu..times.T.sub.r =F.times.R Equation 1
The following equation 2 can then be obtained by taking into account the mechanical loss and the electrical loss. Here, K1, K2, A and B are constants where A=K1.times.F and B=K2. Mechanical losses include losses such as those at the reel motor, losses at the gear 4 in transmitting the driving power from the reel motor to the reels 1 and 2 and losses the reels 1 and 2, while electrical losses include errors such as those generated by errors at the current detection circuit. ##EQU1##
As becomes apparent from equation 2, the tape tension may be held constant by changing the motor current in accordance with the diameter R of the tape roll This tape roll diameter R can be obtained from the rotational period of the reels and the tape speed, as is disclosed in Japanese Publication No. SHO 58-17992 Equation 3 can be obtained using the tape roll diameter of the supply reel RS, the rotational period of the supply reel TS, the diameter of the tape roll on the take reel RT, the rotational period of the take reel TT, the tape speed V and the overall area of the tape 8 and the reel hub AA. Although this will nut be explained in detail, in the equation for the relationship between these items if the rotational periods of the reel TS and TT, and the speed V are known, the diameters of the tape rolls RS and RT can be calculated. EQU AA=.pi.RS.sup.2 +.pi.RT.sup.2 Equation 3 EQU V=2.pi.RS/TS
V=2.pi.RT/TT
FIG. 2 is a view of the tape tension control apparatus for providing control in such a manner that the tape tension F is kept constant by controlling a current I flowing to the reel motor 3 for driving the reel 1 in the reverse system so as to correspond to the tape roll diameter R.
In FIG. 2, the numeral 11 indicates a microcomputer which is supplied with frequency signals SFG and TFG from, a frequency generator (not shown in the diagram) attached between the supply reel 1 and the take reel 2, along with a frequency signal CFG provided from a signal generator (not shown in the diagram) attached to the capstan 6. The microcomputer 11 then obtains the rotational periods TS and TT of the reels from the frequency signals SFG and TFG, the tape speed V is obtained from the frequency signal CFG, and the diameters RS and RT of the tape rolls are calculated.
At the microcomputer 11, the tape roll diameter RS of the supply roll 1 is used as the tape roll diameter R and a current I corresponding to this tape roll diameter R is obtained. This calculated value is then sent to the PWM circuit 12 inside the microcomputer 11 as the control signal SCi. A PWM signal corresponding to the control signal SCi is then outputted from the PWM circuit 12 to be smoothed by a low pass filter 13 before being sent to the fixed current drive circuit 14. In this way, a current I which corresponds to the tape roll diameter R can be sent to the reel motor 15 which drives the reel 1 and the tape tension F can be kept constant.
If the system is almost ideal, simply by controlling the current I flowing to the reel motor 15 so as to be in accordance with the diameter R of the tape roll as in the example in FIG. 2, the tape tension F can be held stable.
There are, however, fluctuations in the reeling in speed of the supply reel 1 due to changes in the load on the drive systems for items such as the reel motor 15 and external disturbances and these may cause fluctuations in the tape tension F or drooping in the tape 8.
Also, if the tape tension F is controlled just by controlling the current I flowing to the reel motor 15 as in the example in FIG. 2, if the tape then sags during transport the rotational speed will be increased as the load on the reel motor 15 is small and the tape 8 will be reeled in a jerking movement. As the sagging in the tape will then have been removed, excessive transit tension will then be instantly applied to the tape 8.
While keeping the tape tension F constant by having a current I which is deduced from equation 2 and corresponds to the diameter of the tape roll R flowing to the reel motor 15, if there are mechanical losses such as losses at the reels 1 and 2, losses at the reel motor 15 or losses at the gear 4, or if there is torque loss, so that there are changes with time the tape tension F will no longer be controlled so as to be constant, i. e. if the tape tension F becomes small when the torque loss increases it will no longer be possible to obtain an output from the desired head. Alternatively, if the tape tension F becomes large when the torque loss is reduced tape damage caused by, for example, the tape guides, will occur.
A ROM look-up table is usually used by the microcomputer 11 to obtain a current I corresponding to the diameter R of the tape roll, i.e. the tape roll diameter R and the tape speed V are taken as input addresses for the ROM and the current I is then obtained by this ROM using the approximation equation 2.
In this case, since average values are used for the constants A and B in the approximation equation 2, fluctuations will occur for each setting for controlling the tape tension. A ROM having a large storage capacity is also required in order to store the tape roll diameters R and the tape speeds V as input parameters.